(1) Field of the Invention
The present invention relates to a fuel battery, particularly to a cylindrical solid electrolyte type fuel battery. The fuel battery can be defined as a battery in which an oxidative reaction is electrochemically carried out, and free energy change attendant on this oxidative reaction is taken out from a cathode and an anode as electrical energy directly.
(2) Description of the Prior Art
As well known, the ionic conductivity of the electrolyte in a high-temperature solid electrolyte type fuel battery is much lower, as compared with that of an alkali type, a phosphoric type or a molten carbonate type fuel battery. In order to assure the performance of the fuel battery, therefore, it is essential to render an electrolytic membrane thin, and at present, its thickness is within the range of 10 to 500 .mu.m.
As a material for electrodes, an Ni base material is selected in view of conditions such as electrolytes, an operating gas and the like, but the electrodes made from such a material have large electric resistance. In consequence, it is also desired to develop the electrodes comprising a thin layer.
The fuel battery is still on the way to development, but for the above mentioned reasons, the most of the conventional fuel batteries have structures shown in FIGS. 3 to 5. That is, as seen in FIG. 3, in many cases, an anode 2, an electrolyte 3 and a cathode 4, each of which is made from the thin layer having a thickness of about 300 .mu.m or less, are formed on a cylindrical porous support pipe 1. In FIG. 4 in which a circular portion A in FIG. 3 is enlargedly shown, reference numerals 5a and 5b represent intermediate connectors, and numeral 6 is a connecting member.
The structure of another fuel battery by conventional technique is illustrated in FIGS. 6 and 7. With regard to this technique, the anode 2 and the cathode 4 are disposed inversely, but they basically have the same structure as in the fuel batterv shown in FIG. 5. In FIG. 6, widths represented by reference symbols L1 and L2 are 0.1 cm and 0.5 cm, respectively. In FIG. 7, reference numeral 7 is a (solid) electrolyte, numeral 8 is a gap between fuel electrodes, 9 is an intermediate connector, and 10 is a gap (0.1 cm) between air electrodes.
The voltage of each unit cell in such a fuel battery is within the range of 0.6 to 1.0 volt. For the purpose of obtaining a high voltage, the unit cells should be connected in series by the use of the intermediate connectors 5a, 5b (or 9) and, in a certain case, connecting members. Further, a large current can be obtained by connecting the sets of unit cells in parallel which have been connected in series.
The performance of the fuel battery depends on an output voltage, and one example of the relation between the latter and an output current is shown in FIG. 8. This drawing indicates that when the output current is increased, a loss will increase and the performance will deteriorate.
The loss content can be classified into various components such as entropy change loss, cathode activating energy loss, anode activating energy loss, electric resistance loss and ion mobile resistance loss. Which loss is greater in a fuel battery depends on the kind of fuel battery and its usable range.
In the high-temperature solid electrolyte type fuel battery, the entropy change loss, the electric resistance loss and the ion mobile resistance loss are larger.
The entropy change loss is greatly dependant on an operating temperature, and thus the decrease in the entropy change loss cannot be desired. Inversely, the entropy change loss is not always considered to be a drawback, because a high-temperature exhaust gas from the fuel battery can lead to the improvement of efficiency in a bottoming cycle.
Therefore, in the high-temperature solid electrolyte type fuel battery, the point to be improved is that the resistance loss attendant on the migration of electricity (electrons) and ions (0-) is great.
This large resistance loss would be due to the following two causes: The first cause is that a specific resistance of the material itself is high, and the second cause is that a current density is not uniform and an effective sectional area of a current channel is decreased by the flow of concentrated current in a particular region, which lead the resistance loss on the whole to increase.
In connection with the phosphoric type and the molten carbonate type fuel batteries which are thought to be put into practice in the near future, the ununiformed current density results from the gas stream. Accordingly, the uniformization of the current density can be achieved by controlling the gas stream.
On the contrary, in the case of the high-temperature solid electrolyte type fuel battery which is different therefrom in the concept of a laminate structure because of having a cylindrical shape, the ununiformed current density will occur, even if the gas stream is controlled well.
FIG. 9 shows current densities in the cathode and the anode of the fuel battery shown in FIG. 5. For convenience, a certain position in the cathode or the anode will be represented hereinafter by an angle. For example, in FIG. 3, a vertical line (not shown) extending through the center of the intermediate connector from the center of the cylindrical cell is regarded as a base line (i.e., 0 or 360 degrees), and the certain position in the cathode or the anode will be represented by an angle between this base line and a line extending through the above certain position in the electrode from the center of the cell. In this way, the angle which is an abscissa axis in FIG. 9 represents the certain position in the electrode. In this drawing, a curve A denotes the current density of the cathode (fuel electrode) and a curve B denotes that of the anode (air electrode).
Since the anode 2 is connected to a connecting piece 6 at a position of 0 degree with the interposition of the intermediate connectors (output terminals on the side of the anode) 5a, 5b, all the current which flows through the fuel battery passes through these portions. The current which has flowed through the anode 2 migrates to the cathode through an electrolyte, therefore the current density in the anode 2 becomes lower with distance from the connecting piece 6. That is, the current density in the anode 2 is maximum at 0 degree and minimum at 180 degrees therein.
Since the cathode 4 is connected to the connecting piece (an output terminal on the side of the cathode) 6 at a position of 180 degrees, all the current which flows through the fuel battery passes through the portion of the cathode 4 which is in contact with the connecting piece 6. On the contrary, the current which passes through other portions of the cathode 4 is only a part of the current which flows through the fuel battery. Therefore, in the cathode 4, the current density is maximum at 180 degrees and is minimum in the vicinity of 0 degree.
FIG. 10 shows a current density distribution in the anode of the fuel battery shown in FIG. 7. In FIG. 10, the length of the unit cell in an axial direction is normalized, to one. In this drawing, 0 and 1 denote the direction of the cathode (fuel electrode) and the direction of the anode (air electrode), respectively. A parameter is the number of cells per stack, i.e., the number of the cells per unit length of the battery.
The results in FIG. 10 indicate that the more the number of the cells per stack is, the more uniform the current density is.